1. Field of the Invention
The invention relates generally to the field of electromagnetic induction well logging instruments and methods. More particularly, the invention relates to methods for correcting multiaxial induction measurements for effects of being eccentered in a wellbore, and for determining dip of rock formations and their resistivity in directions along and transverse to the bedding planes of such formations.
2. Background Art
Electromagnetic induction well logging has as a purpose the determination of electrical resistivity of rock formations. Electrical resistivity is related to parameters of interest of such formations, including fractional volume of pore space of the formation and the fluid content of the pore spaces. Generally, electromagnetic induction well logging includes moving an instrument along a wellbore drilled through rock formations. The instrument includes one or more transmitter antennas (typically in the form of wire coils) and one or more receiver antennas (also typically in the form of wire coils). Alternating current is passed through the transmitter(s) and signals are detected from the receiver(s) related to induced voltages. Characteristics of the induced voltages, for example, amplitude and phase with respect to the transmitter current, are related to the electrical resistivity of the rock formations. Typical induction logging instruments include a plurality of transmitters and receivers spaced apart from each other at selected distances along the length of the instrument so that characteristics of the rock formations may be investigated at a plurality of lateral distances (“depths of investigation”) from the center of the wellbore.
Electromagnetic induction instruments and methods of interpreting the measurements made therefrom include a device used to provide services under the trademark RT SCANNER, which is a trademark of the assignee of the present invention.
The foregoing instrument includes a plurality of triaxial antennas. Each of the triaxial antennas has a wire coil arranged so that its magnetic dipole moment is along the longitudinal axis of the instrument, and two additional, substantially collocated wire coils arranged so that their dipole moments are substantially perpendicular to the axis of the instrument, and substantially perpendicular to each other. One of the triaxial antennas is used as the transmitter, and a plurality of triaxial coils used as receiver antennas are spaced along the instrument at selected longitudinal distances from the transmitter.
An important purpose for the foregoing induction well instrument is to be able to determine resistivity of rock formations both parallel to the direction of layers of the rock formation (“bedding planes”) and in directions perpendicular to the bedding planes. It is known in the art that certain rock formations consist of a plurality of layers of porous, permeable rock interleaved with layers of substantially impermeable rock including substantial volume of clay minerals. Such formations, referred to as “laminated” formations, have been known to be productive of hydrocarbons and have quite different resistivity parallel to the bedding planes as contrasted with perpendicular to the bedding planes.
An important part of interpreting measurements from the foregoing instrument is to correct the measurements for the effects of the wellbore (which occupies some of the volume of investigation of the various receivers) and for the effects of formations having bedding planes disposed at angles other than perpendicular to the axis of the wellbore. Methods known in the art for determining resistivity of such formations using multiaxial electromagnetic induction measurements are described, for example, in U.S. Pat. No. 6,556,015 issued to Omeragic et al. and assigned to the assignee of the present invention, which describes systems and methods for determining subsurface formation properties using an antenna system disposed within a borehole traversing the formation. A logging system described in the foregoing patent includes a well tool implemented with an antenna system having transverse or tilted magnetic dipoles. One antenna implementation uses a set of three coils having non-parallel axes. Through mechanical or electromagnetic rotation of an antenna about its axis in order to maximize or minimize couplings, the systems and methods are used to estimate formation anisotropic resistivity, providing reduced borehole effects in vertical and small deviation wells.
Other techniques are described, for example, in U.S. Pat. No. 6,969,994 issued to Minerbo et al., also assigned to the assignee of the present invention. Methods described in the foregoing patent include disposing a logging instrument in a wellbore, in which the instrument includes a first transmitter antenna having its magnetic moment oriented in a first direction with respect to the instrument axis, a first receiver antenna having its magnetic moment oriented in said first direction with respect to the instrument axis, a second transmitter antenna having its magnetic moment oriented in a second direction with respect to the instrument axis, and a second receiver antenna having its magnetic moment oriented in said second direction with respect to the instrument axis. The method includes activating the first transmitter antenna to transmit electromagnetic energy, measuring a signal associated with the transmitted energy at the second receiver antenna, deactivating the first transmitter antenna; activating the second transmitter antenna to transmit electromagnetic energy, measuring a signal associated with the transmitted energy at the first receiver antenna and calculating the difference between the measured signals to determine a formation property.
There continues to be a need for improved interpretation techniques for multiaxial electromagnetic induction well logging measurements.